RFR: 8229258: Make markOopDesc AllStatic
Roman Kennke
rkennke at redhat.com
Wed Aug 7 21:59:35 UTC 2019
Oh yeah! My IDE completed some operation on markOopDesc to something
that is actually in oopDesc (and would map to something else in
markOopDesc) more than once and left me wondering. E.g.
oopDesc::is_forwarded() which would be markOopDesc::is_marked(). Calling
mark->is_forwarded() seems the obvious thing, but is totally foobared.
So yeah, this is *very* welcome. I guess the changes are alright and
mostly mechanical, but I'll take a deeper look tomorrow, when I'm not
tired ;-)
Thanks & cheers,
Roman
> Hi all,
>
> Please review this patch to make markOopDesc AllStatic.
>
> https://cr.openjdk.java.net/~stefank/8229258/webrev.01/
> https://bugs.openjdk.java.net/browse/JDK-8229258
>
> From the RFE:
>
> I want to change/rename markOop and markOopDesc
>
> Today markOopDescs inherits from oopDesc even though they are not
> oopDescs (Java objects):
> `class markOopDesc : public oopDesc {`
>
> This is very confusing to new-comers (and probably to old-timers as well).
>
> A simple fix would be to break the inheritance and do the following rename:
> markOopDesc -> MarkWord
> markOop -> markWord
>
> However, there are more dubious code in this area. markOopDescs are
> created and used like this:
>
> ```
> class oopDesc {
> ...
> volatile markOop _mark;
> ...
> markOop oopDesc::mark_raw() const { return _mark; }
> ...
> // Usage
> obj->mark_raw()->is_marked()
> ...
> // Implementation
> bool is_marked() const {
> return (mask_bits(value(), lock_mask_in_place) == marked_value);
> }
> ...
> // Implementation
> bool is_being_inflated() const { return (value() == 0); }
> ...
> // Implementation of markOopDesc::value()
> uintptr_t value() const { return (uintptr_t) this; }
> ```
>
> Remember, _mark is an arbitrary bit pattern describing the object. We
> treat it as if it were a pointer to a markOopDesc object, but it's not
> pointing to such an object. While using that (weird) pointer we call
> value() and extract it's bit pattern to be used for further bit pattern
> checking functions. AFAICT, this is also undefined behavior. At least
> is_being_inflated() const is effectively 'return (uintptr_t) this ==
> NULL'. This UB was recently discussed here:
> https://mail.openjdk.java.net/pipermail/hotspot-dev/2019-July/038704.html
> https://mail.openjdk.java.net/pipermail/hotspot-dev/2019-July/038712.html
>
> I propose that we change MarkWord (markOopDesc) to be an AllStatic class
> instead and get rid of this (ab)use of the markWord (markOop) pointer.
>
> The patch is large but mostly straight forward. However, there is some
> Java code that would need some extra scrutiny: SA and JVMCI.
>
> Tested with tier1-3.
>
> Coleen has asked for file rename from markOop.hpp to markWord.hpp. I'd
> like to do that as a separate RFE when/if this patch gets accepted.
>
> Thanks,
> StefanK
>
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